Defining Fibre

Dietary fibre is defined as edible carbohydrate polymers with three or more monomeric units including non-starch polysaccharides, oligosaccharides, and resistant starch.1,2 Unlike other macronutrients, such as fats, proteins and simple carbohydrates, fibre is resistant to the action of mammalian digestive enzymes and is fermented primarily in the large intestine by bacteria of the gastrointestinal (GI) microbiome.

Fibre can have ranging physiochemical properties meaning that its physiological effects upon the host can be widely variable, therefore fibre requires further definition to help categorise individual sources. Fibre is often categorised based on fermentability, solubility or viscosity,3 with solubility referring to the ability to dissolve in water, whereas fermentability refers to the rate of microbial fermentation. Many soluble fibres are also highly fermentable (and vice versa),4 therefore the terms are often used interchangeably.

Fibre and the Microbiome

Dietary fibre can largely impact the composition, diversity and richness of the microbiome, acting as a substrate for specific microbes that possess the necessary enzymes for fermentation of these complex carbohydrates.2 Fibres that demonstrate an ability to specifically or selectively stimulate the growth of beneficial micro-organisms to positively influence microbiome composition and host health are termed ‘prebiotics’.³ For example, fructo-oligosaccharide and acacia gum have shown to increase levels of beneficial bacteria (e.g. Bifidobacteria and Lactobacillus) and reduce potential pathogens (e.g. Clostridium perfringens) in humans, dogs and cats.5-8 Prebiotic fibres are readily fermented resulting in maximal production of beneficial metabolites including lactate and short-chain fatty acids (SCFAs).

Short-chain Fatty Acids

SCFAs are metabolites produced through the microbial fermentation of fibre, primarily butyrate, acetate, and propionate. SCFAs reduce luminal intestinal pH acting to suppress the growth of pathogens and offer a competitive advantage to beneficial bacterial species, promoting a more favourable microbiome composition.9,10 SCFAs also enhance mineral absorption and reduce degradation of peptides into toxic compounds (e.g. ammonia, amines, phenolic compounds).¹¹ Butyrate acts as the preferred energy source for colonocytes, providing approximately 70–80% of their total energy requirement,¹² and is vital for the maintenance of epithelial barrier integrity. Butyrate acts as a key regulator for normal cell colonocyte renewal, enhances intestinal mucin production, promotes epithelial tight junction formation, and is a key messenger molecule, helping to regulate local and systemic immune responses.^12–17

The importance of these metabolites becomes even more apparent when their association with disease is studied. In humans, reduced levels of faecal and intestinal SCFAs, and SCFA-producing bacteria (e.g. Faecalibacterium prausnitzii and Roseburia intestinalis), have been observed in patients with inflammatory bowel disease.18 As such, research is underway to help harness this metabolomic and microbiome data in order to develop biomarkers that can predict disease onset.19 Similarly, dogs with chronic enteropathies possess lower concentrations and abnormal patterns of faecal SCFAs, as well as reductions in important SCFA-producing bacteria (e.g. Blautia spp., Faecalibacterium spp.), decreased bacterial diversity and higher dysbiosis index.20 Further research is required to fully ascertain cause and effect, however this is a promising field for future diagnostic testing and therapeutic targets.